1. Field of the Invention
The present invention relates to electrochemical fuel cell stacks, and, more particularly, to an integrated current collector and electrical component plate for an electrochemical fuel cell stack.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst, disposed at the interfaces between the electrolyte and the electrodes, typically induces the desired electrochemical reactions at the electrodes. The location of the electrocatalyst generally defines the electrochemically active area of the fuel cell.
Polymer electrolyte membrane (PEM) fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two electrode layers comprising a porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth, as a fluid diffusion layer. In a typical MEA, the electrode layers provide structural support to the ion-exchange membrane, which is typically thin and flexible. The membrane is ion conductive (typically proton conductive), and also acts as a barrier for isolating the reactant streams from each other. Another function of the membrane is to act as an electrical insulator between the two electrode layers. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®.
As noted above, the MEA further comprises an electrocatalyst, typically comprising finely comminuted platinum particles disposed in a layer at each membrane/electrode layer interface, to induce the desired electrochemical reactions. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
In a fuel cell, the MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates act as current collectors and provide support for the electrodes. To control the distribution of the reactant fluid streams to the electrochemically active area, the surfaces of the plates that face the MEA may have open-faced channels formed therein. Such channels define a flow field area that generally corresponds to the adjacent electrochemically active area. Such separator plates, which have reactant channels formed therein, are commonly known as flow field plates.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given separator plate may serve as an anode plate for one cell and the other side of the plate may serve as the cathode plate for the adjacent cell. In this arrangement, the plates may be referred to as bipolar plates. The fuel cell stack is typically held together in its assembled state by tie rods and end plates. A compression mechanism is generally required to ensure sealing around internal stack manifolds and flow fields, and also to ensure adequate electrical contact between the surfaces of the plates and membrane electrode assemblies to provide the serial electrical connection among the fuel cells which make up the stack.
Typically, in fuel cell systems, current is drawn from the fuel cell stack via a pair of current collector or bus plates, typically formed of copper or coated copper, one of which is positioned at each end of the fuel cell stack between the assembled fuel cells and the end plates. In order to minimize power losses, the bus plates presently employed in fuel cell systems are typically quite thick (e.g., on the order of 2 mm for an automotive sized stack, but the thickness would be expected to vary somewhat depending on fuel cell size). However, this results in both high through-plane and in-plane thermal conductivity. A consequence of such high through-plane thermal conductivity is that heat is removed from the fuel cell stack, and a consequence of such high in-plane conductivity is that the thermal gradients within the fuel cells in the vicinity of the bus plates are decreased. Both of these consequences may lead to operating issues, such as flooding, in the fuel cells in the vicinity of the bus plates.
Prior attempts to mitigate these operating issues have primarily involved the incorporation of an additional heating component, such as an electric heater or resistive heating element, between the bus plates and end plates of a fuel cell stack (see, e.g., Japanese Patent Publication No. 8-167424, U.S. Patent Application Publication No. 2001/0036568, and U.S. Patent Application Publication No. 2004/0137295). Further additional components, such as high potential bleed down resistors and thermal and electrical insulation layers, have also been incorporated into fuel cell systems to improve performance. However, the presence of these additional components has the disadvantage of increasing the complexity of the fuel cell system design, of increasing the fuel cell system space requirements and of increasing the weight of the fuel cell system. These are significant disadvantages in mobile applications such as fuel cell powered motor vehicles.
Accordingly, although there have been advances in the field, there remains a need in the art for improved fuel cell systems generally, and, in particular, for simple, space-efficient and lightweight fuel cell systems. The present invention addresses these needs and provides further related advantages.